Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. Protein kinases, containing a similar 250-300 amino acid catalytic domain, catalyze the phosphorylation of target protein substrates.
The kinases may be categorized into families by the substrates in the phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Tyrosine phosphorylation is a central event in the regulation of a variety of biological processes such as cell proliferation, migration, differentiation and survival. Several families of receptor and non-receptor tyrosine kinases control these events by catalyzing the transfer of phosphate from ATP to a tyrosine residue of specific cell protein targets. Sequence motifs have been identified that generally correspond to each of these kinase families [Hanks et al., FASEB J., (1995), 9, 576-596; Knighton et al., Science, (1991), 253, 407-414; Garcia-Bustos et al., EMBO J., (1994), 13:2352-2361). Examples of kinases in the protein kinase family include, without limitation, abl, Akt, bcr-abl, Blk, Brk, Btk, c-kit, c-Met, c-src, c-fms, CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8, CDK9, CDK10, cRaf1, CSF1R, CSK, EGFR, ErbB2, ErbB3, ErbB4, Erk, Fak, fes, FGFR1, FGFR2, FGFR3, FGFR4, FGFR5, Fgr, flt-1, Fps, Frk, Fyn, Hck, IGF-1R, INS-R, Jak, KDR, Lck, Lyn, MEK, p38, PDGFR, PIK, PKC, PYK2, ros, Tie, Tie-2, TRK, Yes, and Zap70.
Studies indicated that protein kinases play a central role in the regulation and maintenance of a wide variety of cellular processes and cellular function. For example, kinase activity acts as molecular switches regulating cell proliferation, activation, and/or differentiation. Uncontrolled or excessive kinase activity has been observed in many disease states including benign and malignant proliferation disorders as well as diseases resulting from inappropriate activation of the immune system (autoimmune disorders), allograft rejection, and graft vs host disease.
It is reported that many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease and hormone-related diseases. In addition, endothelial cell specific receptor PTKs, such as VEGF-2 and Tie-2, mediate the angiogenic process and are involved in supporting the progression of cancers and other diseases involving uncontrolled vascularization. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
One kinase family of particular interest is the Src family of kinases. Src kinase is involved in proliferation and migration responses in many cell types, cell activation, adhesion, motility, and survival, growth factor receptor signaling, and osteoclast activation (Biscardi et al., Adv. Cancer Res. (1999), 76, 61-119; Yeatman et al., Nat. Rev. Cancer (2004), 4, 470-480; Owens, D. W.; McLean et al., Mol. Biol. Cell (2000), 11, 51-64). Members of the Src family include the following eight kinases in mammals: Src, Fyn, Yes, Fgr, Lyn, Hck, Lck, and Blk (Bolen et al., Annu. Rev. Immunol, (1997), 15, 371). These are nonreceptor protein kinases that range in molecular mass from 52 to 62 kD. All are characterized by a common structural organization that is comprised of six distinct functional domains: Src homology domain 4 (SH4), a unique domain, SH3 domain, SH2 domain, a catalytic domain (SH1), and a C-terminal regulatory region (Brown et al., Biochim Biophys Acta (1996), 1287, 121-149; Tatosyan et al. Biochemistry (Moscow) 2000, 65, 49-58). SH4 domain contains the myristylation signals that guide the Src molecule to the cell membrane. This unique domain of Src proteins is responsible for their specific interaction with particular receptors and protein targets (Thomas et al., Annu Rev Cell Dev Biol (1997), 13, 513-609). The modulating regions, SH3 and SH2, control intra—as well as intermolecular interactions with protein substrates which affect Src catalytic activity, localization and association with protein targets (Pawson T., Nature (1995), 373, 573-580). The kinase domain, SH1. found in all proteins of the Src family, is responsible for the tyrosine kinase activity and has a central role in binding of substrates. The N-terminal half of Src kinase contains the site(s) for its tyrosine phosphorylation and regulates the catalytic activity of Src (Thomas et al., Annu Rev Cell Dev Biol (1997), 13: 513-609). v-Src differs from cellular Src (c-Src) on the basis of the structural differences in C-terminal region responsible for regulation of kinase activity.
The prototype member of the Src family protein tyrosine kinases was originally identified as the transforming protein (v-Src) of the oncogenic retrovirus, Rous sarcoma virus, RSV (Brugge et al., Nature (1977), 269, 346-348); Hamaguchi et al. (1995), Oncogene 10: 1037-1043). Viral v-Src is a mutated and activated version of a normal cellular protein (c-Src) with intrinsic tyrosine kinase activity (Collett et al., Proc Natl Acad Sci USA (1978), 75, 2021-2024). This kinase phosphorylates its protein substrates exclusively on tyrosyl residues (Hunter et al., Proc Natl Acad Sci USA (1980), 77, 1311-1315).
Investigations indicated that Src is a cytoplasmic protein tyrosine kinase, whose activation and recruitment to perimembranal signaling complexes has important implications for cellular fate. It has well-documented that Src protein levels and Src kinase activity are significantly elevated in human breast cancers (Muthuswamy et al., Oncogene, (1995), 11, 1801-1810); Wang et al., Oncogene (1999), 18, 1227-1237; Warmuth et al., Curr. Pharm. Des. (2003), 9, 2043-2059], colon cancers (Irby et al., Nat Genet (1999), 21, 187-190), pancreatic cancers (Lutz et al., Biochem Biophys Res Commun (1998), 243, 503-508], certain B-cell leukemias and lymphomas (Talamonti et al., J. Clin. Invest. (1993), 91, 53; Lutz et al., Biochem. Biophys. Res. (1998), 243, 503; Biscardi et al., Adv. Cancer Res. (1999), 76, 61; Lynch et al., Leukemia (1993), 7, 1416; Boschelli et al., Drugs of the Future (2000), 25(7), 717), gastrointestinal cancer (Cartwright et al., Proc. Natl. Acad. Sci. USA, (1990), 87, 558-562 and Mao et al., Oncogene, (1997), 15, 3083-3090), non-small cell lung cancers (NSCLCs) (Mazurenko et al., European Journal of Cancer, (1992), 28, 372-7), bladder cancer (Fanning et al., Cancer Research, (1992), 52, 1457-62), oesophageal cancer (Jankowski et al., Gut, (1992), 33, 1033-8), prostate and ovarian cancer (Wiener et al., Clin. Cancer Research, (1999), 5, 2164-70), melanoma and sarcoma (Bohlen et al., Oncogene, (1993), 8, 2025-2031; tatosyan et al., Biochemistry (Moscow) (2000), 65, 49-58). Furthermore, Src kinase modulates signal transduction through multiple oncogenic pathways, including EGFR, Her2/neu, PDGFR, FGFR, and VEGFR (Frame et al., Biochim. Biophys. Acta (2002), 1602, 114-130; Sakamoto et al., Jpn J Cancer Res, (2001), 92: 941-946).
Thus, it is anticipated that blocking signaling through the inhibition of the kinase activity of Src will be an effective means of modulating aberrant pathways that drive oncologic transformation of cells. Src kinase inhibitors may be useful anti-cancer agents (Abram et al., Exp. Cell Res., (2000), 254, 1). It is reported that inhibitors of src kinase had significant antiproliferative activity against cancer cell lines (M. M. Moasser et al., Cancer Res., (1999), 59, 6145; Tatosyan et al., Biochemistry (Moscow) (2000), 65, 49-58).) and inhibited the transformation of cells to an oncogenic phenotype (R. Karni et al., Oncogene (1999), 18, 4654). Furthermore, antisense Src expressed in ovarian and colon tumor cells has been shown to inhibit tumor growth (Wiener et al., Clin. Cancer Res., (1999), 5, 2164; Staley et al., Cell Growth Diff. (1997), 8, 269). Src kinase inhibitors have also been reported to be effective in an animal model of cerebral ischemia (Paul et al. Nature Medicine, (2001), 7, 222), suggesting that Src kinase inhibitors may be effective at limiting brain damage following stroke. Suppression of arthritic bone destruction has been achieved by the overexpression of CSK in rheumatoid synoviocytes and osteoclasts (Takayanagi et al., J. Clin. Invest. (1999), 104, 137). CSK, or C-terminal Src kinase, phosphorylates and thereby inhibits Src catalytic activity. This implies that Src inhibition may prevent joint destruction that is characteristic in patients suffering from rheumatoid arthritis (Boschelli et al., Drugs of the Future (2000), 25(7), 717).
It is well documented that Src-family kinases are also important for signaling downstream of other immune cell receptors. Fyn, like Lck, is involved in TCR signaling in T cells (Appleby et al., Cell, (1992), 70, 751). Hck and Fgr are involved in Fcγ receptor signaling leading to neutrophil activation (Vicentini et al., J. Immunol. (2002), 168, 6446). Lyn and Src also participate in Fcγ receptor signaling leading to release of histamine and other allergic mediators (Turner, H. and Kinet, J-P Nature (1999), 402, B24). These-findings suggest that Src family kinase inhibitors may be useful in treating allergic diseases and asthma.
Other Src family kinases are also potential therapeutic targets. Lck plays a role in T-cell signaling. Mice that lack the Lck gene have a poor ability to develop thymocytes. The function of Lck as a positive activator of T-cell signaling suggests that Lck inhibitors may be useful for treating autoimmune disease such as rheumatoid arthritis (Molina et al., Nature, (1992), 357, 161).
Hck is a member of the Src protein-tyrosine kinase family and is expressed strongly in macrophages, an important HIV target cell and its inhibition in HIV-infected macrophages might slow disease progression (Ye et al., Biochemistry, (2004), 43 (50), 15775-15784).
Hck, Fgr and Lyn have been identified as important mediators of integrin signaling in myeloid leukocytes (Lowell et al., J. Leukoc. Biol., (1999), 65, 313). Inhibition of these kinase mediators may therefore be useful for treating inflammation (Boschelli et al., Drugs of the Future (2000), 25(7), 717).
It is reported that Syk is a tyrosine kinase that plays a critical role in the cell degranulation and eosinophil activation and Syk kinase is implicated in various allergic disorders, in particular asthma (Taylor et al., Mol. Cell. Biol. (1995), 15, 4149).
BCR-ABL encodes the BCR-AEL protein, a constitutively active cytoplasmic tyrosine kinase present in 90% of all patients with chronic myelogenous leukemia (CML) and in 15-30% of adult patients with acute lymphoblastic leukemia (ALL). Numerous studies have demonstrated that the activity of BCR-ABL is required for the cancer causing ability of this chimeric protein.
Src kinases play a role in the replication of hepatitis B virus. The virally encoded transcription factor HBx activates Src in a step required for propagation of the virus (Klein et al., EMBO J. (1999), 18, 5019; Klein et al., Mol. Cell. Biol. (1997), 17, 6427). Some genetic and biochemical data clearly demonstrate that Src-family tyrosine kinases serve as a critical signal relay, via phosphorylation of c-Cbl, for fat accumulation, and provide potential new strategies for treating obesity (Sun et al., Biochemistry, (2005), 44 (44), 14455-14462). Since Src plays a role in additional signaling pathways, Src inhibitors are also being pursued for the treatment of other diseases including osteoporosis and stroke (Susva et al., Trends Pharmacol. Sci. (2000), 21, 489-495; Paul et al., Nat. Med. (2001), 7, 222-227).
It is also possible that inhibitors of the Src kinase activity are useful in the treatment of osteoporosis (Soriano et al., Cell (1991), 64, 693; Boyce et al. J Clin. Invest (1992), 90, 1622; Owens et al., Mol. Biol. Cell (2000), 11, 51-64), T cell mediated inflammation (Anderson et al., Adv. Immunol. (1994), 56, 151; Goldman, F D et al. J. Clin. Invest. (1998), 102, 421), and cerebral ischemia (Paul et al. Nature Medicine (2001), 7, 222).
In addition, src family kinases participate in signal transduction in several cell types. For example, fyn, like lck, is involved in T-cell activation. Hck and fgr are involved in Fe gamma receptor mediated oxidative burst of neutrophils. Src and lyn are believed to be important in Fc epsilon induced degranulation of mast cells, and so may play a role in asthma and other allergic diseases. The kinase lyn is known to be involved in the cellular response to DNA damage induced by UV light (Hiwasa et al., FEBS Lett. (1999), 444, 173) or ionizing radiation (Kumar et al., J Biol Chein, (1998), 273, 25654). Inhibitors of lyn kinase may thus be useful as potentiators in radiation therapy.
T cells play a pivotal role in the regulation of immune responses and are important for establishing immunity to pathogens. In addition, T cells are often activated during inflammatory autoimmune diseases, such as rheumatoid arthritis, inflammatory bowel disease, type I diabetes, multiple sclerosis, Sjogren's disease, myasthenia gravis, psoriasis, and lupus. T cell activation is also an important component of transplant rejection, allergic reactions, and asthma.
T cells are activated by specific antigens through the T cell receptor, which is expressed on the cell surface. This activation triggers a series of intracellular signaling cascades mediated by enzymes expressed within the cell (Kane et al. Current Opinion in Immunol. (2000), 12, 242). These cascades lead to gene regulation events that result in the production of cytokines, like interleukin-2 (IL-2). IL-2 is a necessary cytokine in T cell activation, leading to proliferation and amplification of specific immune responses.
Therefore, Src kinase and other kinase have become intriguing targets for drug discovery (Parang et al., Expert Opin. Ther. Pat. (2005), 15, 1183-1207; Parang et al., Curr. Opin. Drug Discovery Dev. (2004), 7, 630-638). Many classes of compounds have been disclosed to modulate or, more specifically, inhibit kinase activity for use to treat kinase-related conditions or other disorders. For example, U.S. Pat. No. 6,596,746 and the PCT WO 05/096784A2 disclosed benzotrianes as inhibitors of kinases; the PCT WO 01/81311 disclosed substituted benzoic acid amides for the inhibition of angiogenisis; U.S. Pat. No. 6,440,965, disclosed substituted pyrimidine derivatives in the treatment of neurodegenerative or neurological disorders; PCT WO 02/08205 reported the pyrimidine derivatives having neurotrophic activity; PCT WO 03/014111 disclosed arylpiperazines and arylpiperidines and their use as metalloproteinase inhibiting agents; PCT WO 03/024448 described compounds as inhibitors of histone deacetylase enzymatic activity; PCT WO 04/058776 disclosed compounds which possess anti-angiogenic activity. PCT WO 01/94341 and WO 02/16352 disclosed Src kinase inhibitors of quinazoline derivatives. PCT WO03/026666A1 and WO03/018021A1 disclosed pyrimidinyl derivatives as kinase inhibitors. U.S. Pat. No. 6,498,165 reported Src kinase inhibitor compounds of pyrimidine compounds. Peptides as Src Tyrosine Kinase Inhibitors is reported recently (Kumar et al., J. Med. Chem., (2006), 49 (11), 3395-3401). The quinolinecarbonitriles derivatives was reported to be potent dual Inhibitors of Src and Abl Kinases (Diane et al., J. Med. Chem., (2004), 47 (7), 1599-1601).
Another kinase family of particular interest is the aurora kinases. The Aurora kinase family is a collection of highly related serine/threonine kinase that are key regulators of mitosis, essential for accurate and equal segtion of genomic material from parent to daught cells. Members of the Aurora kinase family include three related kinases known as Aurora-A, Aurora-B, and Aurora-C. Despite significant sequence homology, the localization and functions of these kinases are largely distinct from one another (Richard D. Carvajal, et al. Clin Cancer Res 2006; 12(23): 6869-6875; Daruka Mahadevan, et al. Expert Opin. Drug Discov. 2007 2(7): 1011-1026).
Aurora-A is ubiquitously expressed and regulates cell cycle events occurring from late S phase through M phase, including centrosome maturation (Berdnik D, et al. Curr Biol 2002; 12:640-7), mitotic entry (Hirota T, et al. Cell 2003; 114:585-98; Dutertre S, et al. J Cell Sci 2004; 117:2523-31), centrosome separation (Marumoto T, et al. J Biol Chem 2003; 278:51786-95), bipolar-spindle assembly (Kufer T A, et al. J Cell Biol 2002; 158:617-23; Eyers P A, et al. Curr Biol 2003; 13:691-7.), chromosome alignment on the metaphase plate (Marumoto T, et al. J Biol Chem 2003; 278:51786-95; Kunitoku N, et al. Dev Cell 2003; 5:853-64.), cytokinesis (Marumoto T, et al. J Biol Chem 2003; 278:51786-95), and mitotic exit. Aurora-A protein levels and kinase activity both increase from late G2 through M phase, with peak activity in prometaphase. Once activated, Aurora-A mediates its multiple functions by interacting with various substrates including centrosomin, transforming acidic coiled-coil protein, cdc25b, Eg5, and centromere protein A.
Aurora-B is a chromosomal passenger protein critical for accurate chromosomal segregation, cytokinesis (Hauf S, et al. J Cell Biol 2003; 161:281-94; Ditchfield C, et al. J Cell Biol 2003; 161:267-80; Giet R, et al. J Cell Biol 2001; 152:669-82; Goto H, et al. J Biol Chem 2003; 278:8526-30), protein localization to the centromere and kinetochore, correct microtubule-kinetochore attachments (Murata-Hori M, et al. Curr Biol 2002; 12:894-9), and regulation of the mitotic checkpoint. Aurora-B localizes first to the chromosomes during prophase and then to the inner centromere region between sister chromatids during prometaphase and metaphase (Zeitlin S G, et al. J Cell Biol 2001; 155:1147-57). Aurora-B participates in the establishment of chromosomal biorientation, a condition where sister kinetochores are linked to opposite poles of the bipolar spindle via amphitelic attachments. Errors in this process, manifesting as a merotelic attachment state (one kinetochore attached to microtubules from both poles) or a syntelic attachment state (both sister kinetochores attached to microtubules from the same pole), lead to chromosomal instability and aneuploidy if not corrected before the onset of anaphase. The primary role of Aurora-B at this point of mitosis is to repair incorrect microtubule-kinetochore attachments (Hauf S, et al. J Cell Biol 2003; 161:281-94; Ditchfield C, et al. J Cell Biol 2003; 161:267-80; Lan W, et al. Curr Biol 2004; 14:273-86.). Without Aurora-B activity, the mitotic checkpoint is compromised, resulting in increased numbers of aneuploid cells, genetic instability, and tumorigenesis (Weaver B A, et al. Cancer Cell 2005; 8:7-12).
Aurora-A overexpression is a necessary feature of Aurora-A-induced tumorigenesis. In cells with Aurora-A overexpression, mitosis is characterized by the presence of multiple centrosomes and multipolar spindles (Meraldi P et al. EMBO J 2002; 21:483-92.). Despite the resulting aberrant microtubule-kinetochore attachments, cells abrogate the mitotic checkpoint and progress from metaphase to anaphase, resulting in numerous chromosomal separation defects. These cells fail to undergo cytokinesis, and, with additional cell cycles, polyploidy and progressive chromosomal instability develop (Anand S, et al. Cancer Cell 2003; 3:51-62).
The evidence linking Aurora overexpression and malignancy has stimulated interest in developing Aurora inhibitors for cancer therapy. In normal cells, Aurora-A inhibition results in delayed, but not blocked, mitotic entry, centrosome separation defects resulting in unipolar mitotic spindles, and failure of cytokinesis (Marumoto T, et al. J Biol Chem 2003; 278:51786-95). Encouraging antitumor effects with Aurora-A inhibition were shown in three human pancreatic cancer cell lines (Panc-1, MIA PaCa-2, and SU.86.86), with growth suppression in cell culture and near-total abrogation of tumorigenicity in mouse xenografts (Hata T, et al. Cancer Res 2005; 65:2899-905.).Aurora-B inhibition results in abnormal kinetochore-microtubule attachments, failure to achieve chromosomal biorientation, and failure of cytokinesis (Goto H, et al. J Biol Chem 2003; 278:8526-30; Severson A F, et al. Curr Biol 2000; 10:1162-71). Recurrent cycles of aberrant mitosis without cytokinesis result in massive polyploidy and, ultimately, to apoptosis (Hauf S, et al. J Cell Biol 2003; 161:281-94; Ditchfield C, et al. J Cell Biol 2003; 161:267-80; Giet R, et al. J Cell Biol 2001; 152:669-82; Murata-Hori M, Curr Biol 2002; 12:894-9; Kallio M J, et al. Curr Biol 2002; 12:900-5).
Inhibition of Aurora-A or Aurora-B activity in tumor cells results in impaired chromosome alignment, abrogation of the mitotic checkpoint, polyploidy, and subsequent cell death. These in vitro effects are greater in transformed cells than in either non-transformed or non-dividing cells (Ditchfield C, et al. J Cell Biol 2003; 161:267-80). Thus, targeting Aurora may achieve in vivo selectivity for cancer. Although toxicity to rapidly dividing cell of the hematopoietic and gastrointestinal system is expected, the activity and clinical tolerability shown in xenograft models indicates the presence of a reasonable therapeutic index. Given the preclinical antitumor activity and potential for tumor selectivity, several Aurora kinase inhibitors have been developed. The first three small-molecule inhibitors of Aurora described include ZM447439 (Ditchfield C, et al. J Cell Biol 2003; 161:267-80), Hesperadin (Hauf S, et al. J Cell Biol 2003; 161:281-94), and MK0457 (VX680) (Harrington E A, et al. Nat Med 2004; 10:262-7). The following agents are nonspecific inhibitors: ZM447439 inhibits Aurora-A and Aurora-B; Herperadin inhibits primarily Aurora-B; MK0457 inhibits all three Aurora kinases. Each induces a similar phenotype in cell-based assays, characterized by inhibition of phosphorylation of histone H3 on Ser10, inhibition of cytokinesis, and the development of polyploidy. Selective inhibitors of Aurora have also been developed. A selective Aurora-A inhibitor is MLN8054 (Hoar H M, et al. [abstract C40]. Proc AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics 2005). A example of selective Aurora-B inhibitor is AZD1152 (Schellens J, et al. [abstract 3008]. Proc Am Soc Clin Oncol 2006; 24:122s). The next generation of Aurora inhibitors is currently being developed, including agents by Nerviano Medical Sciences (PHA-680632 and PHA-739358), Rigel (R763), Sunesis (SNS-314), NCE Discovery Ltd. (NCED #17), Astex Therapeutics (AT9283), and Montigen Pharmaceuticals (MP-235 and MP-529). Several of these agents are undergoing evaluation in clinical trials.
Many cancers are characterized by disruptions in cellular signaling pathways that lead to uncontrolled growth and proliferation of cancerous cells. Receptor tyrosine kinases (RTKs) play a crucial role in these signaling pathways, transmitting extracellular molecular signals into cytoplasm and/or nucleus of a cell. RTKs are transmembrane proteins that generally include an extracellular ligand-binding domain, a membrane-spanning domain and a catalytic cytoplasmic tyrosine kinase domain. The binding of ligand to the extracellular potion is believed to promote dimerization, resulting in trans-phosphorylation and activation of the intracellular tyrosine kinase domain (Schlessinger et al. Neuron 1992; 9:383-391).
Another kinase family of particular interest is FLT3. FMS-related tyrosine kinase 3 (FLT3), also known as FLK-2 (fetal liver kinase 2) and STK-1 (human stem cell kinase 1), belongs to a member of the class Ill receptor tyrosine kinase (RTKIII) family that include KIT, PDGFR, FMS and FLT1 (Stirewalt D L, et al. Nat. Rev. Cancer 2003; 3:650-665; Rosnet O, et al. Genomics 1991; 9:380-385; Yarden Y, et al. Nature 1986; 323: 226-232; Stanley E R, et. al. J. Cell. Biochem. 1983 21:151-159; Yarden Y, et al. EMBO J 1987; 6:3341-3351). FLT3 is a membrane-spanning protein and composed of four domains; an extracellular ligand-binding domains consisting of five immunoglobin-like structures, a transmembrane (TM) domain, a juxtamembrane (JM) domain and a cytoplasmic C-Terminal tyrosine kinase (TK) domain. (Agnes F, et al. Gene 1994; 145:283-288; Scheijen B, et al. Oncogene 2002; 21:3314-3333).
The ligand for FLT3 (FLT3 or FL) was cloned in 1993 and shown to be a Type I transmembrane protein expressed in cells of the hematopoietic bone marrow microenvironment, including bone marrow fibroblasts and other cells (Lyman S D, et al. Cell 1993; 75:1157-1167). Both the membrane-bound and soluable forms can activate the tyrosine kinase activity of the receptor and stimulate growth of progenitor cells in the marrow and blood. Binding of ligand to receptor induces dimerisation of the receptor and activation of the kinase domains; which then autophosphorylate and catalyse phosphorylation of substrate proteins of various signal transduction pathways such as signal transducer and activator of transcription 5 (STATS), RAS/mitogen-activated protein kinase (RAS/MAPK), phosphoinositide 3-kinase (PI3K), src homologous and collagen gene (SHC), SH2-containing inositol-5-phosphatase (SHIP), and cytoplasmic tyrosine phosphatase with 2 Src-homology 2 (SH2) domains (SHP2), which play important roles in cellular proliferation, differentiation, and survival (Dosil M, et al. Mol Cell Biol 1993; 13:6572-6585. Zhang S, Biochem Biophys Res Commun 1999; 254:440-445). In addition to hemotopoietic cells, FLT3 gene is also expressed in placenta, gonads and brain (Maroc N, et al. Oncogene 1993; 8:909-918) and also plays an important role in the immune response (deLapeyriere O, et al. Leukemia 1995; 9:1212-1218).
FLT3 is overexpressed at the levels in 70-100% of cases of acute myeloid leukemias (AML), and in a high percentage of T-acute lymphocytic leukemia (ALL) cases (Griffin J D, et al. Haematol J. 2004; 5:188-190). It is also overexpressed in a smaller subset of chronic myeloid leukemia (CML) in blast crisis. Studies have shown that the leukemic cells of B lineage ALL and AML frequently co-express FL, setting up autocrine or paracrine signaling loops that result in the constitutive activation of FLT3 (Zheng R, et. al. Blood. 2004; 103:267-274).
Evidence is rapidly accumulating that many types of leukemias and myeloproliferative syndromes have mutation in tyrosine kinases. FLT3 mutations are one of the most frequent somatic alterations in AML, occurring in approximately ⅓ of patients. There are two types of activating mutations in FLT3 described in patients with leukemia. These include a spectrum of internal tandem duplications (ITD) occurring within the auto-inhibitory juxtamembrane domain (Nakao M, et al. Leukemia 1996; 10:1911-1918; Thiede C, et al. Blood 2002; 99:4326-4335), and activation loop mutations that include Asp835Tyr (D835Y), Asp835Val (D835V), Asp835His (D835H), Asp835Glu (D835E), Asp835Ala (D835A), Asp835Asn (D835N), Asp835 deletion and Ile836 deletion (Yamamoto Y, et al., Blood 2001:97:2434-2439; Abu-Duhier F M, et al. Br. J. Haematol. 2001; 113:983-988). Internal tandem duplication (ITD) mutations within the JM domain contribute to about 17-34% of FLT3 activating mutations in AML. FLT3-ITD has also been detected at low frequency in myelodysplastic syndrome (MDS) (Yokota S, et al. Leukemia 1997; 11:1605-1609; Horiike S, et al. Leukemia 1997; 11:1442-1446). The ITDs are always in-frame, and are limited to the JM domain. However, they vary in length and position from patient to patient. These repeat sequences may serve to disrupt the autoinhibitory activity of the JM domain resulting in the constitutive activation of FLT3. Both FLT3-ITD and FLT3-Asp835 mutations are associated with FLT3 autophosphorylation and phosphorylation of downstream targets (Mizuki M, et al. Blood 2000; 96:3907-3914; Mizuki M, et al. Blood 2003; 101:3164-3173; Hayakawa F, et al. Oncogene 2000; 19: 624-631).
Inhibitors of FLT3 are presently being studied and have reached clinical trials as monotherapy in relapsed or refractory AML patients, some or all of whom had FLT3 mutations. FLT3 inhibitors, such as PKC412 (N-benzoyl staurosporine) (Fabbro D, et al. Anticancer Drug Des 2000; 15:17-28; Weisberg E, et al. Cancer Cell 2002; 1:433-443), CT53518 (also known as MLN518) (Kelly L M, et al. Cancer Cell 2002; 1:421-432), SU11248 (O'Farrell A M, et al. Blood 2003; 101:3597-3605), SU5614 (Spiekermann K, et al. Blood 2003; 101:1494-1504), and SU5416 (Giles F J, et al. Blood 2003; 102:795-801), have been shown to have antitumor activity. Collectively, these data suggest that FLT3 is an attractive therapeutic target for the development of kinase inhibitors for AML and other associated diseases.
Considering the lack of currently available treatment options for the majority of the conditions associated with protein kinases, there is still a great need for new therapeutic agents that inhibit these protein targets. Particularly, Aurora kinase inhibitors are of special interest in treating certain disorders, including cancer.